In recent years, from the viewpoints of global environment protection and economical efficiency, compact self-ballasted fluorescent lamps with electrodes, which are about five times higher in efficiency in comparison with incandescent lamps and also have an operating life time about six times longer than that of incandescent lamps, have been widely used in houses, hotels and the like in place of incandescent lamps. Moreover, recently, in addition to conventionally-used compact self-ballasted fluorescent lamps with electrodes, electrodeless compact self-ballasted fluorescent lamps have been utilized. Since the electrodeless fluorescent lamp, which has no electrodes, has an operating life time that is about two times longer than that of a fluorescent lamp with electrodes, it is expected to spread more and more in the future.
Conventionally, incandescent lamps having various shapes have been devised and put into practical use, and those having a pyriform shape have been most widely used. This shape is defined as A-type in JIS C7710-1988, and is also defined in the same manner internationally in IEC 60887-1988, and in accordance with this standard, similar standards have been set in the United States, Europe, etc. Most of lighting devices for lighting incandescent lamps have been prepared on the premise to be used for these A-type incandescent lamps. For this reason, with respect to the compact self-ballasted fluorescent lamps also, in particular, there have been demands for practically providing the shape and the size similar to those of A-type incandescent lamps.
The size of the generally-used A-type incandescent lamp is set to 60 mm in diameter and 110 mm in height from the top of the bulb to the tip of the base, for example, in the case of the incandescent lamp of 100 W in input power, and in order to replace incandescent lamps, it is important to determine the size of the compact self-ballasted fluorescent lamp so as not to excessively exceed the above-mentioned size.
Different from the incandescent lamp, the fluorescent lamp converts ultraviolet emitted by mercury that has been excited by electric discharge into visible light through a phosphor layer applied onto an external-tube bulb (bulb); thus, the fluorescent lamp functions as a light source. Among the ultraviolet emitted by mercury, in particular, that having luminescent line emission with a wavelength of 253.7 nm has the highest conversion efficiency to visible light in the phosphor layer. In other words, the efficiency of a fluorescent lamp is determined by the radiation efficiency of ultraviolet luminescent line of 253.7 nm. This efficiency in the fluorescent lamp is determined by the number density in mercury atoms inside the lamp, that is, the vapor pressure, and the highest efficiency is achieved in the case of about 6 m Torr (about 798 mPa). This state corresponds to the saturated vapor pressure at about 40° C. of the mercury droplet. For this reason, in an attempt to design a fluorescent lamp having high efficiency, it is desirable to set the temperature of at least a portion of the external-tube bulb to have the lowest temperature (hereinafter, referred to as the coldest point) to the vicinity of 40° C. Thus, excessive mercury vapor is allowed to form droplets at the coldest point.
Here, in general, in the case of a compact self-ballasted fluorescent lamp to be used for substituting an incandescent lamp, the size of the lamp is smaller for the power to be supplied to the lamp in comparison with a tublar fluorescent lamp. For this reason, upon operating, the temperature of the bulb becomes higher, and it is difficult in principle to set the temperature of the bulb to the vicinity of 40° C. In other words, in comparison with the straight tube fluorescent lamp and the like, the compact self-ballasted fluorescent lamp has a greater power per unit surface area, with the result that heat radiation from the lamp surface is not carried out sufficiently to cause a high temperature in the bulb.
With respect to the countermeasures to these problems, for example, Japanese Patent Application Laid-Open No. 11-31476 has proposed a method in which amalgam is used. In this method, by allowing amalgam to adsorb excessive mercury vapor that exceeds the optimal value due to a temperature rise upon operation, the mercury vapor pressure at the time of operation is controlled to the vicinity of the optimal value, and Bi—In based and Bi—Pb—Sn based amalgams, which have a mercury-vapor-pressure-controlling function, are utilized in this method.
Further, Japanese Patent Application Laid-Open No. 2001-325920 has proposed another countermeasure in which, a bump portion is formed at a portion to have the lowest temperature in a bulb toward the outside of the bulb so that heat radiation is locally increased so as to set the temperature of the corresponding portion to the vicinity of 40° C.
In the method using the amalgam, however, in the case when a lamp is turned on from a turn-off state in which the lamp temperature is low, since it takes some time until the amalgam has had a temperature rise to again release the adsorbed mercury, the resulting problem is that it takes not less than several minutes of rising-time to obtain sufficient brightness from the lamp after the turning-on.
Moreover, in the case of a method in which, in order to shorten the rising-time of brightness, without using amalgam, the bump portion is formed on the outer wall of the bulb with mercury droplets being enclosed in the bulb, although the effect for controlling the temperature of the coldest point to the vicinity of 40° C. is obtained, the glass strength of the bump portion tends to weaken to be easily broken. Furthermore, since the incandescent lamp has no bump portion of this type, it is not desirable from the aesthetic viewpoint, when this fluorescent lamp is used in place of an incandescent lamp.
The present invention has been devised to solve the above-mentioned problems, and its main objective is to provide a compact self-ballasted electrodeless discharge lamp which controls the temperature of the coldest point within a desired range by using a technique that is different from the conventional techniques, and an electrodeless-discharge-lamp lighting device.